: a light architectural member resting on purlins and usually running at right angles to them.

What is the role of purlin?

Purlin plates are beams which support the mid-span of rafters and are supported by posts. By supporting the rafters they allow longer spans than the rafters alone could span, thus allowing a wider building.

What is a purlin vs Rafter?

Both rafters and purlins are beams. And they both occur mostly in roofs. The difference is that, if you stand facing the ridge of a roof, purlins span left to right, and rafters span down to up. Rafters can also be trussed rafters, so they might form a triangular shape and have triangulated sections within.